feat: prevent deletion of seeded entity schemas

[themusicman]

Summary

• Add a guard to DeleteEntity that rejects attempts to delete the schema of seeded entity types (people, tasks, notes, links, interests, skills)
• Add Seeds.seeded_type?/1 helper to check if a type is seeded
• Add comprehensive tests for both the guard and the helper

Detailed Changes

Backend

• Brain.Seeds — New seeded_type?/1 function that checks membership against entity_types/0
• DeleteEntity — Pre-delete guard that blocks schema-targeted deletions (__schema__, schema, type name, <type>.json) for seeded types while still allowing normal entity deletions within those types
• Brain.Helpers — New :protected_entity_type error formatting with the offending type name

Tests

• SeedsTest — seeded_type?/1 returns true for all 6 seeded types, false for custom types and nil
• BrainActionsTest — Rejection of seeded schema deletion, successful entity deletion within seeded types, successful deletion in custom types

Test plan

• seeded_type?/1 returns true for all seeded types
• seeded_type?/1 returns false for non-seeded values
• Deleting a seeded type schema target is rejected with clear error
• Deleting a normal entity within a seeded type still works
• Deleting entities in custom (non-seeded) types still works